Line replaceable unit control valve

ABSTRACT

A control valve for a hydraulic actuator assembly is constructed in a manner holding certain dimensions therein within predetermined limits so that the valves are interchangeable without recalibration. In addition, the valves are constructed to be easily removed from a hydraulic actuator assembly with a straight line motion which does not adversely effect the calibration of the control valve and of the actuator assembly.

United States Patent Wheeler 1 1 July 18, 1972 s41 LINE REPLACEABLE UNITCONTROL 2,409,842 10/1946 Eaton ..91/3s4 VALVE 2,766,732 10/1956Schu1tz.. ..91/384 2,988,881 6 1961 R ..91 384 72 Inventor: Mildred s.Wheeler, Long Beach, Calif.

[73] Assignee: McDonnell Douglas Corporation Primary ExaminerPaul E.Maslousky [22] Filed: y 13 1970 gaging-Walter .1. Jason, Donald L. Royerand George W.

[21] Appl. No.: 54,197

[ ABSTRACT [52] U.S.Cl. ..9l/384,91/367,137/315, A t l v lv f r ahydraulic actuator assembly is con- 251/366 structed in a manner holdingcertain dimensions therein [5 l Int. Cl. ..Fl5b 9/10 wihin predeterminedlimits so that the valves are interchange [58] Field of Search ..91/384,367; 251/366; 137/315 able without recalibration In addition, the valvesare com structed to be easily removed from a hydraulic actuator as- [56]References C'ted sembly with a straight line motion which does notadversely ef- UNTED STATES PATENTS feet the calibration of the controlvalve and of the actuator assembly. 2,301,176 11/1942 Elliott ..251/3663,348,804 10/1967 Piccardo ..137/315 4Clains,4Drawlng Figures-W////////////,,.z'l

PATENTEDJUUBIBR 3,877,139

sum 1 OF 2 I NVENTOR.

LINE REPLACEABLE UNIT CONTROL VALVE BACKGROUND OF THE INVENTION Modernday aircraft are now of such size and/or fly at such airspeeds thatpower boosts through the use of hydraulic actuator assemblies on thecontrol surfaces as well as other portions thereof are required. Thehydraulic actuator assemblies employed in aircraft while normally beingreliable and efficient, have certain operative portions thereof which,due to various problems, have relatively low life expectancies incomparison to the hydraulic actuator assemblies overall. An example ofsuch portions are the control valves thereof which have replacementrates of from 3 to 5 times the replacement rate of the manifold orcylinder assemblies to which they are attached. This is true for alltypes of control valves from those controlled manually having a simple,single input up to redundant types requiring complex electrical andmechanical multiple inputs with summing linkages. None of theseheretofore known control valves have been designed specifically to bereplaceable or removable from the manifold or cylinder assemblies towhich they are attached while installed in the aircraft and therefore,failures of such control valves have necessitated removal of entirehydraulic actuator assemblies. On most aircraft in use today, theremoval of the entire hydraulic actuator assembly is difficult and timeconsuming. In addition, some of the current generation of aircraftrequire hydraulic actuator assemblies which are so large in size andweight that the removal thereof present extreme maintenance problemsrequiring the aid of external hoists and in most cases, the removal ofseveral stressed access doors in the aircraft skin.

It is therefore desirable to construct control valves which are linereplaceable units (LRU). Conversion of the heretofore available controlvalves into LRU type control valves has been found impossible. Forexample, the single input control valves now in use require the removalof a valve crank or difficult lateral movement combined with pullingmotion to disengage the valve drive arm. In addition, many controlvalves require the removal of the housing for the valve and valve spoolseparately while control valves with complex summing linkages such asare used in conjunction with autopilots, require the opening of sideplates in the manifolds thereof to disengage the summing linkage. Mostcontrol valves also require the removal of a large diameter, threadedretaining ring requiring application of high torque in a restricted areawithin the aircraft. All of these procedures mentioned above can andusually do, destroy the sensitive null or zero flow calibration betweenthe control valve and the rest of the hydraulic actuator assembly and inaddition, expose the assembly and most particularly the control valve toenvironmental contamination and the loss of expensive hydraulic fluid.When the null calibration is destroyed, the entire assembly must beremoved from the aircraft and recalibrated on a hydraulic flow stand, avery expensive and time consuming operation. When contamination isintroduced into the assembly, the reliability and the life expectancy,not only of the assembly but of the entire hydraulic system, isadversely effected.

SUMMARY OF THE INVENTION The present invention overcomes these and otherdisadvantages and shortcomings of the previously known control valves byproviding a line replaceable unit (LRU) control valve which due to novelconstruction including the holding of certain predetermined tolerancestherein, can be easily removed and replaced from an associated hydraulicactuator assembly without destroying the null calibration thereof andcan also be interchanged with other control valves, because such valvescan be precisely null calibrated separately from the rest of thehydraulic assembly. The predetermined tolerances involved includeholding the position of a plurality of attachment points or bolt holeswhich are placed at specified locations in the control valve housingwith respect to linkage abutment surfaces within the valve which arecalibrated to control the operation thereof. Generally by holding thetolerances between these portions of the control valves, constructedaccording to the present invention, the valves can be madeinterchangeable and replaceable in hydraulic actuator assemblies withoutrecalibration thereof.

Therefore, a principle object of the present invention is to provide acontrol valve with interchangeability without recalibration bycontrolling the tolerances thereof through the use of locating diametersor pins so that the valves can be removed, replaced and spared asseparate LRUs from the actuator or manifold assemblies.

Another object is to provide an LRU control valve which permits theremoval of the valve from the actuator or manifold it controls in astraight line movement by merely removing attachment bolts.

Another object is to provide an LRU control valve where the chances ofthe control spool therein binding are minimized.

Another object is to provide an LRU control valve which is relativelyimmune to contamination since the valve parts requiring close lappedtits on assembly are entirely contained within the removable housingportion thereof and are thus protected from contamination. The entry ofcontaminates is therefore confined to interfacing surfaces of the valvehousing itself.

Another object is to provide an LRU control actuator which in designconcept can be used with single system control valves, single systemdual concentric control valves, dual system tandum control valves, anddual system tandem concentric control valves.

Another object is to provide a tandem LRU control valve which meets FAAregulations which state no single failure shall cause the loss of twohydraulic systems.

These and other objects and advantages of the present invention willbecome apparent after considering the following detailed specificationwhich covers various embodiments thereof in conjunction with theaccompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side view of a tandemelectro-hydromechanical servo actuator assembly with its normalorentation shown by arrows.

FIG. 2 is an enlarged top view of a tandem LRU control valve lookingdown into the linkage entrance port thereof and includinG a portion ofthe linkage positioned therein shown in cross-section taken along line2-2 of FIG. 1.

FIG. 3 is a view of the hydraulic actuator assembly of FIG. I from theunderside thereof with the LRU control valve of FIG. 2 installedthereon.

FIG. 4 is an exploded view of the hydraulic actuator assembly of FIGS. 1and 3 including the LRU control valve in cross-section taken along line4a4a of FIG. 2, the hydraulic actuator in cross-section taken along line4b4b of FIG. 3, a servo actuator, a servo lockout and interconnectinglinkage therebetween. The portions of the assembly are not necessarilyto scale.

DESCRIPTION OF THE PRESENT EMBODIMENT Referring to the drawings moreparticularly by reference numbers, number 10 in FIG. I refers to ahydraulic actuator assembly for use as a control surface actuator on anaircraft. The arrows labeled UP and FORWARD adjacent thereto show thenormal orientation of the actuator assembly 10 when installed on anaircraft. The theory of operation of such assemblies is explained inU.S. Pat. application Ser. No. 47,741, filed concurrently herewith byRobert E. Schenbeck entitled Multi-channel Autopilot with EqualizationMeans which is assigned to applicant's assignee.

The assembly 10 includes a hydraulic actuator 12 whose output arm 14provides the connection to the aircraft control surface (not shown) tomove it. Also included in the assembly is an LRU main control valve 16constructed according to the present invention. The control valve 16 isshown in FIG. 2 which is a top view thereof, looking down into thelinkage en trance port 18 in the valve housing 20.

The control valve 16 is connected to the remainder of the actuatorassembly 10 by close tolerance bolts 22 which pass through bolt holes 24in the housing thereof. Also shown in FIG. 2 are the hydraulic manifolds26 and 28 of the valve 16. The valve 16 shown is a tandem valve whichrelatively independently controls the hydraulic flow to either twoactuators or a dual actuator such as actuator 12 of FIG. 1. Themanifolds 26 conduct the hydraulic flow to one portion of the actuator12 and the manifolds 28 conduct the hydraulic flow to the other. Themanifolds 26 and 28 and the linkage port 18 face in an upwardlydirection so that when the valve 16 is removed from the rest of theassembly, the hydraulic fluid remains within the valve 16 to protect itfrom contamination as much as possible. This arrangement also reducesspillage of hydraulic fluid to a minimum.

Referring to FIG. 4 to get a better view of the assembly 10 in FIGS. 1and 3, FIG. 4 is an exploded view of the assembly 10 showing the moreimportant portions thereof including the hydraulic actuator 12, the LRUcontrol valve 16, an autopilot servo 30, a lockout device 32 and linkagemeans 34 to mechanically interconnect the various components of theassembly. Input commands are fed into such an assembly 10 either throughelectrical inputs to the servo 30 or by means of manual mechanicalinputs at pivot point 36 on link 38 which forms a part of the linkagemeans 34. The link 38 connects the output arm 14 of the hydraulicactuator 12, connected thereto by an arm 40, to the main control valve16 by means of another link 42, a crank 44, a shaft 46, and a summinglink 48. The axis of rotation 50 of the shaft 46 as shown, is above thepivotal axis 52 at which point the summing link 48 is pivotallyconnected to the shaft 46.

Although the hydraulic actuator 12, the servo 30, the lockout device 32,and the assorted linkage are specific to one particular hydraulicactuator assembly 10, various types of linkage and actuators can be usedwith LRU control valves embodying the present invention.

In normal autopilot operation of the assembly 10, an electrical inputcommand is fed to the servo 30 which then moves the position of thecontrol valve 16 from its null flow condition to the position producingthe desired flow condition. The servo 30 does this by moving the upperend 54 of the summing link 48 which rotates about pivotal axis 52because the lockout device 32 prohibits the shaft 46 to which it isconnected by arm 55 from rotating. This causes the lower end 56 of thesumming link 48 which extends down into the control valve 16 through thelinkage port 18 to move the spool 60 of the control valve 16. Thiscauses the aforesaid flow of hydraulic fluid to the hydraulic actuator12 which moves the output arm 14 thereof and the connected controlsurface in the desired direction. As the output arm 14 moves, arm 40feeds the mechanical motion thereof back through the link 38 whichrotates about the pivot 62 connecting link 38 to link 42 to therebyreposition the manual control system (not shown) connected to pivotpoint 36 in correspondence to the new position of the control surface.

When manual inputs are used to control the assembly 10, the lockout 32is disengaged so that the shaft 46 can rotate about the axis 50. At thesame time the servo 30 is fixed in a centered position. Manual inputs atpoint 36 therefore cause rotation of the shaft 46 about the axis 50 toreposition the axis 52. Since the upper end 54 of the summing link 48 isrelatively fixed by the centered servo 30, the lower end 56 of thesumming link 48 is moved in proportion to the movement of the axis 52 tomove the spool 60 of the control valve 16. The control valve then allowsthe flow of hydraulic fluid to the ac tuator 12 to move the output arm14 thereof and the connected control surface. The motion of the outputarm 14 is fed back through the arm 40, the link 42, the crank 44 and theshaft 46 to reposition the summing link 48 and the lower end 56 thereofso that the spool 60 of the control valve 16 is recentered and the flowof hydraulic fluid is reduced to zero thereby ceasing motion of theoutput arm 14 and the control surface in the position commanded.

It should be apparent from the above discussion that it is critical thatthe control valve 16 be returned to a null or zero flow condition whenno further travel of the output arm 14 is desired. The zero or nullpoint of such a control valve 16 must be very small or the resolutionand/or repeatability will be poor. In addition, since many functions onmodern aircraft depend upon the accuracy of such a control system, thenull point must also be very accurately calibrated. Heretofore, maincontrol valves such as those which perform the function of LRU controlvalve 16 have required a complete recalibration upon installation in thehydraulic assembly 10. This has been due, in part, to the inability tohold tolerances due to the specific designs thereof and because theprevious assembly designs have not been constructed with ease ofinstallation and removal of the main control valve from the rest of theas sembly in mind.

Looking more specifically at the control valve 16 as shown in FIGS. 2and 4, the valve is mounted to the hydraulic assembly 10 by the closetolerance bolts 22 with the valve spool 60 at a right angle to adirection of attachment. The valve spool 60 as shown includes a fulldiameter groove 64 defined by parallel abutment surfaces 66 and 68. Thesurfaces 66 and 68 are lapped to a very close tolerance both in distanceapart and in parallelism. The lower end 56 of the summing link 48 isalso lapped to a very close diameter tolerance to closely fit within thetwo abutment surfaces 66 and 68. Since the tolerances are held betweenthe abutment surfaces 66 and 68 and the lower end 56 of the summing link48, valves 16 and summing links 48 are interchangeable without beingmatched. When the valve spool 60 is trimmed to null during manufacture,the location of the groove 64 is held in relationship to the closetolerance bolt holes 24 which are the reference points used to interfacethe control valve 16 with the rest of the assembly. When the rest of theassembly 10 is calibrated, with the shaft 46 fixed by the lockout device32, the location of the summing link 48, and therefore axes 50 and 52are held to bolt holes in the assembly (not shown) for the bolts 22which retain the control valve 16 in proper position. The control valve16 when installed is indexed to these holes by its bolt holes 24 and theclose tolerance bolts 22. All of this null calibration of the hydraulicactuator assembly takes place with relationship of the summing link 48to the bolt holes. The control valve 16 is therefore interchangeablewith other control valves without recalibration.

The control valve 16 is removable from the rest of hydraulic assembly bythe removal of the close tolerance bolts 22. When the control valve 16is freed by removal of the bolts 22, it can be removed by a straightdownwardly pull parallel to the direction of bolt attachment so that thelower end 56 of the summing link 48 slides vertically out from betweenthe two abutment surfaces 66 and 68 of the groove 64 in the controlvalve spool 60. A new control valve is installed by reversing theprocedure. The control valve 16 can be pulled vertically down from therest of the assembly 10, as it is normally sits, and therefore thehydraulic fluid in the control valve 16 remains therein and the innerworkings thereof are only subject to contamination from outside sourcesat the openings, of the manifolds 26 and 28 and the linkage entranceport 18.

It should be apparent that although in the tandem valve 16 shown in FIG.2, a single summing link 48 is included, the valve 16 is adaptable todual summing linkages. By looking at FIG. 2, it can be seen that thevalve 16 is adaptable to have the lower end 56 of two summing links 48inserted within the groove 64 on both sides of the spool rather than onthe one side as shown. These types of dual valve linkages are usefulwhen a dual redundance is required so that if one summing link breaks,the assembly 10 is still operable. In regards to these redundancies, thecontrol valve 16 is constructed employing rip stop means so that if aportion of the housing 20 fails by cracking, the crack can not grow"into other portions of the housing 20. These rip stop" means are shownin FIG. 2 and 4 as sections 72 and 74 of the housing 20 which sectionsare constructed of material such as brass through which cracks do notpropagate. The valve 16 as shown can also be easily converted from atandem valve to a single control valve. This can be done by removing theportion 76 of the valve 16 including the enclosed portion of the spool60 between the rip stop" means 72 and 74. It should be noted that all ofthe locating bolt holes 24 of the described embodiment will remain afterthe removal of portion 76 during conversion.

Thus there has been shown and described a novel LRU control valve whichfulfills all the objects and advantages sought therefor including beingable to be line replaceable on a hydraulic assembly withoutrecalibration thereof. Many changes, alterations, modifications, otheruses and applications of the subject LRU control valve will becomeapparent to those skilled in the art after considering thisspecification and the accompanying drawings. All such changes,alterations and modifications which do not depart from the spirit andscope of the invention are deemed to be covered by the invention whichis limited only by the claims which follow.

iclaim:

l. A replaceable valve for controlling the flow of fluid to ac curatelycontrol the positioning of a hydraulic actuator having linkage means tofeed input commands to the valve at predetermined times, said valveincluding:

a valve housing, including means for removably connecting said valvehousing to the actuator in accurate position with respect to apredetermined position of the linkage means thereof, said housing alsoincluding at least two portions which are bonded together by a brazedconnection which precludes cracks in one portion thereof frompropagating to the other portion thereof; and

a valve spool within said housing whose position with respect to saidhousing is related to the control that said valve exerts over the flowof fluid, said valve spool having a groove therein for engagement withthe linkage means, said groove being accurately positioned at apredetermined location in said valve spool with respect to a position ofthe valve spool within said housing which causes cessation of the flowof fluid to the actuator to maintain the actuator in the positioncommanded by the input command; said valve having a predetermined normalorientation with respect to gravity, said connecting means for saidvalve housing allowing straight downward removal of said valve housingfrom the actuator in the normal orientation, said valve housing furtherincluding:

a linkage port through which the linkage means extend; and

hydraulic fluid manifolds which connect to the actuator to providepassageways for the fluid whose flow to the actuator is controlled bysaid valve; said port and manifolds defining openings in the valvehousing which in the normal orientation thereof face upwardly so thatthe fluid therein is retained therein by gravity when said valve isremoved from the actuator to thereby prevent excessive waste of fluidand to protect said valve spool within said housing from contamination.

2. A replaceable valve for controlling the flow of fluid to control ahydraulic actuator having linkage means to feed input commands to thevalve at predetermined times, said valve having a predetermined normalorientation with respect to gravity and including:

a valve housing, including means for removably connecting said valvehousing to the actuator in accurate position with respect to apredetermined position of the linkage means thereof, wherein saidconnecting means allow straight downward removal of said valve housingfrom the actuator in the normal orientation, a linkage port throughwhich linkage means extend, and hydraulic fluid manifolds which connectto the actuator to provide passageways for the fluid whose flow to theactuator is controlled by said valve; and

a valve spool within said housing whose position with respect to saidhousing is related to the control that said valve exerts over the flowof fluid, said valve spool having a longitudinal axis there-along and agroove therein for engagement with the linkage means, said groove beingaccurately positioned at a predetermined location in said valve spoolwith respect to a position of the valve spool within said housing whichcauses cessation of the flow of fluid to the actuator, said groove beingdefined by two parallel abutment walls, perpendicular to saidlongitudinal axis, said abutment walls being in abutment with thelinkage means, said means for connecting said valve housing to saidactuator connecting in a direction parallel to said parallel abutmentwalls so that upon removal of said valve housing from the actuator saidvalve housing can be removed in a direction parallel to said abutmentwalls no matter what the position of said spool within said housing, andsaid port and manifolds defining openings in the valve housing which inthe normal orientation thereof face upwardly so that the fluid thereinis retained therein by gravity when said valve is removed from theactuator to thereby prevent excessive waste of fluid and to protect saidvalve spool within said housing from contamination.

3. The replaceable valve defined in claim 2 wherein said means forremovably connecting said valve housing to the actuator include:

bolt holes formed in said valve housing in accurate predeterminedpositions with respect to said groove when said valve spool is at theposition which causes cessation of the flow of fluid to the actuator;and close tolerance bolts which extend through said bolt holes toremovably connect said valve housing to the actuator.

4. A replaceable valve for controlling the flow of fluid to accuratelycontrol the positioning of a hydraulic actuator having linkage means tofeed input commands to the valve at predetermined times, said valveincluding:

a valve housing, including means for removably connecting said valvehousing to the actuator in accurate position with respect to apredetermined position of the linkage means thereof; and

a valve spool within said housing whose position with respect to saidhousing is related to the control that said valve exerts over the flowof fluid, said valve spool having a groove therein for engagement withthe linkage means, said groove being accurately positioned at apredetermined location in said valve spool with respect to a posi tionof the valve spool within said housing which causes cessation of theflow of fluid to the actuator to maintain the actuator in the positioncommanded by the input command, said valve spool also having alongitudinal axis therealong and wherein said groove is defined by twoparallel abutment walls, perpendicular to said longitudinal axis, saidabutment walls being in abutment with the linkage means and said meansfor connecting said valve housing to said actuator connecting in adirection parallel to said parallel abutment walls so that upon removalof said valve housing from the actuator said valve housing can beremoved in a direction parallel to said abutment walls no matter thatthe position of said spool within said housing, said valve also having apredetermined normal orientation with respect to gravity, saidconnecting means for said valve housing allowing straight downwardremoval of said valve housing from the actuator in the normalorientation, said valve housing further including:

a linkage port through which the linkage means extend; and

hydraulic fluid manifolds which connect to the actuator to providepassageways for the fluid whose flow to the actuator is controlled bysaid valve; said port and manifolds defining openings in the valvehousing which in the normal orientation thereof face upwardly so thatthe fluid therein is retained therein by gravity when said valve isremoved from the actuator to thereby prevent excessive waste of fluidand to protect said valve spool within said housing from contamination.

1. A replaceable valve for controlling the flow of fluid to accuratelycontrol the positioning of a hydraulic actuator having linkage means tofeed input commands to the valve at predetermined times, said valveincluding: a valve housing, including means for removably connectingsaid valve housing to the actuator in accurate position with respect toa predetermined position of the linkage means thereof, said housing alsoincluding at least two portions which are bonded together by a brazedconnection which precludes cracks in one portion thereof frompropagating to the other portion thereof; and a valve spool within saidhousing whose position with respect to said housing is related to thecontrol that said valve exerts over the flow of fluid, said valve spoolhaving a groove therein for engagement with the linkage means, saidgroove being accurately positioned at a predetermined location in saidvalve spool with respect to a position of the valve spool within saidhousing which causes cessation of the flow of fluid to the actuator tomaintain the actuator In the position commanded by the input command;said valve having a predetermined normal orientation with respect togravity, said connecting means for said valve housing allowing straightdownward removal of said valve housing from the actuator in the normalorientation, said valve housing further including: a linkage portthrough which the linkage means extend; and hydraulic fluid manifoldswhich connect to the actuator to provide passageways for the fluid whoseflow to the actuator is controlled by said valve; said port andmanifolds defining openings in the valve housing which in the normalorientation thereof face upwardly so that the fluid therein is retainedtherein by gravity when said valve is removed from the actuator tothereby prevent excessive waste of fluid and to protect said valve spoolwithin said housing from contamination.
 2. A replaceable valve forcontrolling the flow of fluid to control a hydraulic actuator havinglinkage means to feed input commands to the valve at predeterminedtimes, said valve having a predetermined normal orientation with respectto gravity and including: a valve housing, including means for removablyconnecting said valve housing to the actuator in accurate position withrespect to a predetermined position of the linkage means thereof,wherein said connecting means allow straight downward removal of saidvalve housing from the actuator in the normal orientation, a linkageport through which linkage means extend, and hydraulic fluid manifoldswhich connect to the actuator to provide passageways for the fluid whoseflow to the actuator is controlled by said valve; and a valve spoolwithin said housing whose position with respect to said housing isrelated to the control that said valve exerts over the flow of fluid,said valve spool having a longitudinal axis there-along and a groovetherein for engagement with the linkage means, said groove beingaccurately positioned at a predetermined location in said valve spoolwith respect to a position of the valve spool within said housing whichcauses cessation of the flow of fluid to the actuator, said groove beingdefined by two parallel abutment walls, perpendicular to saidlongitudinal axis, said abutment walls being in abutment with thelinkage means, said means for connecting said valve housing to saidactuator connecting in a direction parallel to said parallel abutmentwalls so that upon removal of said valve housing from the actuator saidvalve housing can be removed in a direction parallel to said abutmentwalls no matter what the position of said spool within said housing, andsaid port and manifolds defining openings in the valve housing which inthe normal orientation thereof face upwardly so that the fluid thereinis retained therein by gravity when said valve is removed from theactuator to thereby prevent excessive waste of fluid and to protect saidvalve spool within said housing from contamination.
 3. The replaceablevalve defined in claim 2 wherein said means for removably connectingsaid valve housing to the actuator include: bolt holes formed in saidvalve housing in accurate predetermined positions with respect to saidgroove when said valve spool is at the position which causes cessationof the flow of fluid to the actuator; and close tolerance bolts whichextend through said bolt holes to removably connect said valve housingto the actuator.
 4. A replaceable valve for controlling the flow offluid to accurately control the positioning of a hydraulic actuatorhaving linkage means to feed input commands to the valve atpredetermined times, said valve including: a valve housing, includingmeans for removably connecting said valve housing to the actuator inaccurate position with respect to a predetermined position of thelinkage means thereof; and a valve spool within said housing whoseposition with respect to said housing is related to the control thatsaid valve exerts over the flow of fluid, said valve spool having agroove Therein for engagement with the linkage means, said groove beingaccurately positioned at a predetermined location in said valve spoolwith respect to a position of the valve spool within said housing whichcauses cessation of the flow of fluid to the actuator to maintain theactuator in the position commanded by the input command, said valvespool also having a longitudinal axis therealong and wherein said grooveis defined by two parallel abutment walls, perpendicular to saidlongitudinal axis, said abutment walls being in abutment with thelinkage means and said means for connecting said valve housing to saidactuator connecting in a direction parallel to said parallel abutmentwalls so that upon removal of said valve housing from the actuator saidvalve housing can be removed in a direction parallel to said abutmentwalls no matter that the position of said spool within said housing,said valve also having a predetermined normal orientation with respectto gravity, said connecting means for said valve housing allowingstraight downward removal of said valve housing from the actuator in thenormal orientation, said valve housing further including: a linkage portthrough which the linkage means extend; and hydraulic fluid manifoldswhich connect to the actuator to provide passageways for the fluid whoseflow to the actuator is controlled by said valve; said port andmanifolds defining openings in the valve housing which in the normalorientation thereof face upwardly so that the fluid therein is retainedtherein by gravity when said valve is removed from the actuator tothereby prevent excessive waste of fluid and to protect said valve spoolwithin said housing from contamination.